1. Field of the Invention
This invention relates to an actuator for moving a shaft axially against a high load force, the actuator being powered by a relatively low power motor. It can be used to move any shaft in any situation, but is particularly suitable for moving valves, chokes or other control mechanisms used in e.g. oil or gas production or process plant.
The invention relates particularly to actuators for valves used in remote or inaccessible locations, e.g. sub-sea valves of a sub-sea oil or gas production complex. It will be described with reference to such a sub-sea location, but it is to be understood that it could be used in any terrestrial or aquatic location.
2. DESCRIPTION OF RELATED ART
Currently sub-sea valves are operated by hydraulics or electro-hydraulics. Some valves have a safety system so that they fail-safe if there is any loss of power. They may fail-safe closed or open depending on the valve. A common fail-safe system is one or open valve has a spring tending to move it to a closed (or open) position and a powered actuator acting against the spring. If there is any loss of power, the spring automatically moves the valve to its fail-safe position.
Direct hydraulics for powering the valve actuators are suitable and are extensively used for sub-sea units within a reasonable distance of a surface operating installation. There is a considerable response time for direct hydraulics, and electro-hydraulics more suitable for sub-sea units at any distance from the operating installation. Electric power and signals are transmitted to electrical solenoids which operate hydraulic control valves having a hydraulic fluid supply system at the sub-sea unit. A considerable volume of fluid is required, and, for faster operations, storage accumulators are necessary. Either closed or open hydraulic systems can be used, with failure of the electrical or hydraulic systems allowing the loss of venting off of the hydraulic fluid and allowing the valve to move to the fail-safe position.
The use of hydraulics has the following drawbacks:
there is a time lag between the command given at an operating installation and the actuation, this time lag increasing as the distance from the operating installation increases,
it creates high pressure circuits involving numerous module connections.
moving seals are required which must have long term sealing properties,
the system must be ultra clean to prevent seal damage or cylinder scoring,
there is no direct confirmation of the operation unless electrical travel sensors are installed.
Present sub-sea oil and gas production is limited by these methods of valve operation and control, particularly as regards the distance which the production can be from the operating installation. Costly platforms or floating installations are therefore required even in deep water locations.
Some functions of sub-sea oil and gas production, particularly in running and retrieving modules, cannot readily be operated other than by hydraulics, e.g. module connectors and soft landing jacks. However there would be advantages in operating all production functions electrically, particularly in operating oil valves (including chokes) electrically.
Electrical systems can be built to very high standards of reliability. A high power supply can be run to the sub-sea installation using a compact armoured steel cable with either electrical or fibre optics signal and feedback communication lines.
Due to quick response, all the power can be supplied by the cable (with no hydraulic storage requirements required). This is especially important when controlling a large number of operations e.g. when controlling a sub-sea production installation start up that includes separation and pumping. (See UK Patent Application No. 2202561).